Fuel cells have emerged as one of the most promising new technologies for meeting future global energy needs. In particular, fuel cells that consume hydrogen are proving to be environmentally clean, quiet, and highly efficient devices for power generation. However, while hydrogen fuel cells have a low impact on the environment, the current methods for producing hydrogen require high-temperature steam reforming of non-renewable hydrocarbon fuels. Further still, these high-temperature methods produce significant amounts of polluting emissions and greenhouse gases such as carbon dioxide (CO2).
A key challenge for promoting and sustaining the vitality and growth of the fuel cell industry (as well as the entire industrial sector of society) is to develop efficient and environmentally benign technologies for generating fuel, such as hydrogen, from renewable resources. Notably, if hydrogen fuel for consumption in fuel cells can be generated efficiently from renewable sources, then non-renewable resources such as petroleum feedstocks can be used for other, more beneficial, and less environmentally deleterious purposes. Moreover, the generation of energy from renewable resources such as biomass, reduces the net rate of production of carbon dioxide, an important greenhouse gas that contributes to global warming. This is because the biomass itself, i.e., plant material, consumes carbon dioxide during its life cycle.
At present, the vast majority of hydrogen production is accomplished via steam reforming of a hydrocarbon (usually methane) over a suitable catalyst. Conventional steam reforming takes place at considerably elevated temperatures, generally from about 400° C. to 700° C. or even higher (673 to 937 K and higher).
The net desired steam reformation reaction of a hydrocarbon is shown in reaction (1). The reaction requires a catalyst, conventionally a nickel-based catalyst on a modified alumina support.CxH2x+2+xH2O→xCO+(2x+1)H2  (1)The nickel catalyst is sensitive to sulfur poisoning, which can be problematic. Hydrocarbon feedstocks produced from petroleum contain a significant amount of sulfur. Therefore, the hydrocarbon reactants must have the contaminating sulfur removed prior to undergoing steam reforming.
Conventional steam reforming is generally followed by one or more water-gas shift (WGS) reactions (reaction (2)) that take place in a second and perhaps a third reactor.CO+H2O→CO2+H2  (2)
The WGS reaction uses steam to convert the carbon monoxide produced in reaction (1) to carbon dioxide and hydrogen. The WGS reaction is thus used to maximize the production of hydrogen from the initial hydrocarbon reactants.
An entire, and typical, prior art process for the steam reformation of methane is illustrated schematically in FIG. 1. The hydrocarbon feedstock is first desulfurized at 10. The desulfurized feedstock is then subjected to a first high-temperature, vapor-phase reforming reaction in a first high-temperature reaction chamber at 12. As noted earlier, this reaction generally uses a nickel-based catalyst. The products of the reaction at 12 are then swept into a second reactor for a first WGS reaction 14. This first WGS reaction nnntakes place at approximately 300° C. to 350° C., using an iron catalyst. The products of the reaction at 14 are swept into a third reactor for a second WGS reaction 16. This second WGS reaction takes place a reduced temperature of from about 200° C. to 250° C. The products of the reaction at 16 are then passed through a separator 18, where the products are separated into two streams: CO2 and H2O (the water which is pumped back into the reaction cycle at the beginning) and CO and H2. The CO and H2 stream from the separator 18 may also be subjected (at 20) to a final methanation reaction (to yield CH4 and H2) or an oxidation reaction to yield CO2 and H2.
It has been reported that it is possible to produce hydrogen via steam reformation of methanol at temperatures near 277° C. (550 K). See B. Lindstrom & L. J. Pettersson, Int. J Hydrogen Energy 26(9), 923 (2001), and J. Rostrup-Nielsen, Phys. Chem. Chem. Phys. 3, 283 (2001). The approach described in these references uses a copper-based catalyst. These catalysts, however, are not effective to steam reform heavier hydrocarbons because the catalysts have very low activity for cleavage of C—C bonds. Thus, the C—C bonds of heavier hydrocarbons will not be cleaved using these types of catalysts.
Wang et al., Applied Catalysis A: General 143, 245-270 (1996), report an investigation of the steam reformation of acetic acid and hydroxyacetaldehyde to form hydrogen. These investigators found that when using a commercially available nickel catalyst (G-90C from United Catalysts Inc, Louisville, Ky.), acetic acid and hydroxyacetaldehyde can be reformed to yield hydrogen in high yield only at temperatures at or exceeding 350° C. Importantly, the nickel catalyst was observed to deactivate severely after a short period of time on stream.
A hydrogen-producing fuel processing system is described in U.S. Pat. No. 6,221,117 B1, issued Apr. 24, 2001. The system is a steam reformer reactor to produce hydrogen disposed in-line with a fuel cell. The reactor produces hydrogen from a feedstock consisting of water and an alcohol (preferably methanol). The hydrogen so produced is then fed as fuel to a proton-exchange membrane (PEM) fuel cell. Situated between the reactor portion of the system and the fuel cell portion is a hydrogen-selective membrane that separates a portion of the hydrogen produced and routes it to the fuel cell to thereby generate electricity. The by-products, as well as a portion of the hydrogen, produced in the reforming reaction are mixed with air, and passed over a combustion catalyst and ignited to generate heat for running the steam reformer.
Conventional steam reforming has several notable disadvantages. First, the hydrocarbon starting materials contain sulfur which must be removed prior to steam reformation. Second, conventional steam reforming must be carried out in the vapor phase, and high temperatures (greater than 500° C.) to overcome equilibrium constraints. Because steam reformation uses a considerable amount of water which must also be heated to vaporization, the ultimate energy return is far less than ideal. Third, the hydrocarbon starting materials conventionally used in steam reforming are highly flammable. The combination of high heat, high pressure, and flammable reactants make conventional steam reforming a reasonably risky endeavor.
Thus, there remains a long-felt and unmet need to develop a method for producing hydrogen that utilizes low sulfur content, renewable, and perhaps non-flammable starting materials. Moreover, to maximize energy output, there remains an acute need to develop a method for producing hydrogen that proceeds at a significantly lower temperature than conventional steam reforming of hydrocarbons derived from petroleum feedstocks. Lastly, there remains a long-felt and unmet need to simplify the reforming process by developing a method for producing hydrogen that can be performed in a single reactor.